In this type of the conventional pneumatic bias tire comprising plural pairs of bead cores and plural carcass ply groups corresponding to the bead core pair number, carcass plies of rubberized ply cords constituting each group being wound around each bead core, it is common that when the number of ply cords per 50 mm in at least three inside carcass plies of an innermost carcass ply group is Np, a rubber gauge between the ply cords of the adjoining carcass plies is Tp (mm), a diameter of the ply cord is Dp (mm), the number of ply cords per 50 mm in the carcass ply of the other carcass ply groups is Nq, a rubber gauge between the ply cords of the adjoining carcass plies is Tq (mm) and a diameter of the ply cord is Dq (mm), there are made Np=50, Nq=51 and Np/Nq=0.98 but also Tp/Dp=1.3, Tq/Dq=0.8 and (Tp/Dp)/(Tq/Dq)=1.6.
Moreover, the term “rubber gauge between ply cords of adjoining carcass plies” used throughout the specification means a total thickness of coating rubber sandwiched between the ply cords of mutually laminated carcass plies and rubber which may be separately interposed between the carcass plies.
In such a conventional tire, however, it tends to increase a ratio of causing separation failure between one to three inside carcass plies in the innermost carcass ply group, more directly separation failure of the ply cord from the coating rubber due to the increase of strain in a portion of the carcass corresponding to a position of the maximum tire width accompanied with increases of load and torque for the construction vehicle or the like in recent years.
Namely, in the loader, a maximum width portion of the tire having a thinnest thickness is particularly subjected to a large strain by increasing the load accompanied with the recently large bucketing and the output accompanied with the increase of digging force. Particularly, in the bending deformation of the tire side portion under a large load, the innermost first to third carcass plies are located at a compression side of the bending and the carcass ply as a tension-resistant member is largely distorted by subjecting to compression force instead of tensile force, so that shearing strain between the carcass plies becomes large to cause the separation failure of the ply cord from the coating rubber and hence separation failure between the plies resulted therefrom.
In connection with such problems, JP-A-57-191102 proposes a technique wherein on the assumption that the twisting number of the ply cord in the outermost and innermost carcass ply groups is made larger than the twisting number of the ply cord in the middle carcass ply group to enhance compression strength in these carcass ply groups, the amount of rubber existing between the ply cords is increased for preventing the peeling of the ply cord from the coating rubber due to the increase of the cord twisting number, whereby the concentration of shearing strain is controlled to prevent cracking produced at an interface between the cord and the rubber and hence prevent the separation failure.
Even in such a tire, however, the separation failure of the ply cord from the coating rubber can not be sufficiently prevented when compression force applied to the inner carcass ply is made larger by the bending deformation of the tire side portion as in the bias tire having plural pairs of bead cores obliged to increase the thickness in the portion corresponding to the maximum tire width and also having a ply rating of not less than 58 used under a high load, or when an aspect ratio of the tire is not more than 80% and the bending deformation amount of the tire side portion is made relatively large due to the lowering of the height of the tire side portion or the like.
The invention is to solve the problems of the conventional techniques and to provide a pneumatic bias tire capable of effectively preventing the separation failure between the carcass plies even in the large bending deformation of the tire side portion under an action of a high load.